1. Field of the Invention
The present invention relates to a head-mounted display which displays a video before the eyes of an observer.
2. Description of the Related Art
Recently, head-mounted video displays have been developed which enlarge video displayed on video display devices such as liquid crystal displays, display it before the eyes of an observer, and thereby allow the observer to watch large-screen video.
In a video display disclosed in Japanese Patent Laid-Open No. 11-174988, a pair of LCDs 3 corresponding to the left and right eyes of an observer are placed on the left and right inside an outer frame 2 of a device body 1 as shown in FIG. 6, where the LCDs 3 are two-dimensional video display devices. Backlights 4 are placed behind the respective LCDs 3 to illuminate the LCDs 3 and a pair of prisms 5 are placed below the LCDs 3 to project enlarged video from the LCDs 3 to both eyes E. The video displayed on the LCDs 3 is projected to both eyes E of the observer M along respective observation optical axes L and projected in front of both eyes E by being enlarged when passing through the prisms 5.
To provide a wide viewing angle to the observer, it is necessary to increase magnification of the prism. However, increases in the magnification of the prism make it difficult to maintain optical performance, resulting in marked chromatic aberration, field curvature, and other optical aberrations. Attempts to correct such aberrations properly result in a complicated, bulky optical system, making it difficult to mount the video display properly on the observer.
To deal with such situations, As being disclosed in Japanese Patent Laid-Open No. 5-127660, a technique, for adding distortion to video data before display by processing a video signal to be displayed so that the distortion will be cancelled out after passage through an optical system, is disclosed.
Also, an example disclosed in Japanese Patent Laid-Open No. 8-149393 proposes a configuration for displaying a black frame in peripheral portions to cancel out distortion in shape even though no distortion is added to video to be displayed.
Although this configuration does not eliminate distortion in the displayed video, four sides of the peripheral frame appear to be straight and four corners appear to be square, reducing a sense of image distortion felt by the observer.
Also, Japanese Patent Laid-Open No. 2004-268638 discloses a configuration for a vehicle-mounted video display which projects video onto a front window of a vehicle, wherein frame members are arranged in such a way as to cancel out distortion caused by the front window. Also, with this configuration, although the displayed video remains distorted, four sides of the frame appear to be straight and four corners appear to be square, reducing a sense of image distortion felt by the observer.
However, with the technique disclosed in Japanese Patent Laid-Open No. 5-127660, video distorted by the optical system is observed outside a video-display area. Thus, the technique has a problem in that edges of the video-display area are distorted, resulting in degradation of video quality.
Also, the technique disclosed in Japanese Patent Laid-Open No. 8-149393, has a problem in that a corrective frame shape is displayed in the peripheral portions, reducing the number of display elements actually used for the video, which in turn results in low video resolution.
Also, the technique disclosed in Japanese Patent Laid-Open No. 2004-268638 presents a cost problem because frame members must be shaped to fit the front window of the vehicle on which the frame members are mounted, making it necessary to prepare multiple shapes.
On the other hand, if a visual field other than the video observed by the observer is darkened, the observer can view the video standing out in the visual field as if on a screen of a movie theater, increasing the quality of the observed video.
To provide such video, Japanese Patent No. 3485689 discloses a method for placing a viewing frame in front of a video display device to allow the observer to view video standing out in the visual field fitted entirely in a black frame as if on a screen of a movie theater.
However, with the technique disclosed in Japanese Patent No. 3485689, an aperture of the viewing frame is smaller than an effective display area of the liquid crystal display element, and consequently part of the video displayed on the liquid crystal display element is hidden by the viewing frame. With a configuration in which part of displayed video is hidden by the viewing frame in this way, the entire video is not shown to the observer. Take, for example, a PC monitor screen or the like; there can be a problem if part of video is not displayed. Also, Japanese Patent Laid-Open No. 5-127660 mentions nothing about positioning of viewing frames on the left and right display devices, and thus presents a problem of discrepancy between left and right video display areas.